The present invention relates to the field of molding and is concerned, more particularly, with the molding of articles from fast setting plastics.
There are certain new types of fast setting plastics such as polyurethanes formed by mixing two or more ingredients which interreact and rapidly solidify. Because of the rapid setting characteristics of the plastic, the ingredients cannot be mixed and satisfactorily dispensed into a forming mold by conventional molding techniques. If any of the mixed ingredients remain in the mixing and dispensing head, then an immediate second attempt to expel the mixed ingredients may be required (which is not usually practical), or the head must be flushed immediately with a solvent which is a costly, messy, and toxic operation. Failing to do either of these operations allows the mixed ingredients to set up in the head which leads to shutdown of the molding machine and a costly cleaning operation.
Another problem associated with the fast setting plastics, is that the ingredients must be thoroughly mixed prior to the molding of the plastic in the desired final form. Complete mixing is necessary and must be done in proper proportions in order to allow the ingredients to interreact completely and thus produce the rapid setting plastic.
It will be understood that the two problems, that is forming the fast setting plastic in proper and final form before the plastic sets up and thoroughly mixing the ingredients forming the plastic, are somewhat antagonistic. Thorough mixing of the ingredients takes a finite time, but the time in which the mixed ingredients set up is short. Thus, the mixing apparatus must be particularly efficient in operation and located within a short distance of the mold or other object which determines the final shape and form of the plastic when set up.
Because of the above-described problems, the new fast setting plastics have not been used in conventional molding machines. Nevertheless, the rapid setting features of the plastics are desirable, and the adaption of a molding machine to such plastics is a worthwhile objective.
Accordingly, it is a general object of the present invention to provide both method and apparatus for utilizing rapid setting plastics in a molding machine. A more specific object of the present invention is to provide a mixing valve which rapidly mixes ingredients forming a fast setting plastic and then dispenses all of the mixed ingredients from the valve to keep the valve cleaned out.